Add emoji picker and message reactions (Gitea issue #11)

Composer gains an emoji picker (static curated unicode list, insert at
cursor position) and messages gain Slack/Mattermost-style reactions:
react with any emoji, toggle off by reacting again, see who reacted via
a tooltip on each pill.

Backend: MessageReaction model (unique on message_id+user_id+emoji backs
toggle semantics), WS "reaction" envelope broadcasts the full recomputed
reaction list per message (same approach as message edits), REST message
list embeds reactions so a reload doesn't lose state that only arrived
over WS.

Frontend: shared EmojiPicker component (anchored popover, Escape/outside-
click dismiss via new useEscapeKey hook) used by both the composer and a
new hover-revealed reaction trigger on each message row.

backend/app/routers/rooms.py

@@ -42,7 +42,7 @@ from app.schemas.webhook import (
     WebhookIncomingCreate,
     WebhookIncomingRead,
 )
-from app.services.message_service import list_recent_messages
+from app.services.message_service import get_reactions_for_messages, list_recent_messages
 from app.services.room_service import (
     CannotRemoveOwnerError,
     DuplicateRoomError,
@@ -295,6 +295,7 @@ async def get_room_messages_endpoint(
     require_scope(request, "read:messages")
     await require_room_member(room_id, current_user, db)
     messages = await list_recent_messages(db, room_id, limit)
+    reactions_by_message = await get_reactions_for_messages(db, [m.id for m in messages])
     return [
         MessageRead(
             id=m.id,
@@ -303,6 +304,7 @@ async def get_room_messages_endpoint(
             username=m.user.username,
             content=m.content,
             image_id=m.image_id,
+            reactions=reactions_by_message.get(m.id, []),
             created_at=m.created_at,
             edited_at=m.edited_at,
         )